The present invention relates to a passenger detection system, and in particular, to a passenger detection system which can detect the presence or absence of a passenger seated on a seat of a vehicle which is provided with an air bag unit, seatbelts, etc., by means of a simple method and equipment.
1. Description of the Prior Art
Air bag units for absorbing shocks and providing relief from impact damage of car passengers in cases of collisions have become indispensable gear for car safety, and are being provided not only to driver's seats but also to passenger seats in recent years.
FIG. 1 is a circuit diagram showing a conventional circuit employed in air bag systems. The circuit of FIG. 1 comprises a driver's seat squib circuit composed of a series connection of a safety sensor SS1 such as a mechanical accelerometer, a squib SQ1, and a semiconductor switching device SW1 such as an FET (Field-Effect Transistor), and a passenger seat squib circuit composed of a series connection of a safety sensor SS2, a squib SQ2, and a semiconductor switching device SW2 such as an FET, and an electronic accelerometer (collision sensor) AM for detecting negative acceleration due to collisions, and a control circuit CC provided with functions for judging the presence or absence of a collision according to an output signal of the electronic accelerometer AM and supplying signals to the gates of the semiconductor switching devices SW1 and SW2.
When a car provided with the air bag system collided with something, switches of the safety sensors SS1 and SS2 is closed with relatively small negative acceleration enabling the driver's seat squib circuit and the passenger seat squib circuit. If the control circuit CC judged that the car actually collided with something according to the output signal from the electronic accelerometer AM, the control circuit CC supplies signal to the gates of the semiconductor switching devices SW1 and SW2 and the semiconductor switching devices SW1 and SW2 are turned ON, thereby currents are passed through the two squib circuits. Due to the currents, the squibs SQ1 and SQ2 are heated and thereby the air bags for the driver's seat and the passenger seat are deployed to protect the passengers from impact damage by the collision.
Incidentally, such an air bag system is effective for saving the passenger in the case where an adult person P is seated on the seat 1 as shown in FIG. 2A. However, in the case where an infant SP is seated on an infant seat 1A fixed on the passenger seat facing rear (hereafter referred to as `RFIS (Rear Facing Infant Seat)` as shown in FIG. 2B, deployment of the air bag might hurt the infant, and thus it is preferable that the air bag does not deploy on the collision. Further, in the case where a child SP' is seated on a child seat 1A' fixed on the passenger seat facing forward (hereafter referred to as `FFCS (Forward Facing Child Seat)` as shown in FIG. 2C, deployed air bag might cover the face of the child SP' and suffocate the child SP', and thus it is preferable that the air bag does not deploy on the collision similarly to the case of RFIS.
As a countermeasure against the above problems, a circuit for air bag systems shown in FIG. 3 has been proposed, for example. The circuit of FIG. 3 further comprises a passenger detection sensor device SD for detecting the status of the passenger on the passenger seat. The control circuit CC judges whether or not a passenger is seated on the passenger seat and the status of the passenger on the passenger seat, according to a detection signal from the passenger detection sensor device SD, and sets itself at deployment mode in which the control circuit CC deploys the air bag on collision, or at no deployment mode in which the control circuit CC does not deploy the air bag on collision. As the passenger detection sensor device SD, a device employing a weight sensor and a device employing image processing have been proposed. In the method employing image processing, the passenger is shot by a camera and it is judged whether the passenger is an adult person P or a child SP' or an infant SP by means of image processing.
By the first method employing a weight sensor, whether the passenger is an adult P or a child SP' or an infant SP can roughly be judged and the above unexpected accidents can basically be avoided by the setting of the control circuit CC into the deployment mode or the no deployment mode based on the judgment. However, such a method employing a weight sensor lacks precision since weight varies widely among individuals and there exist cases where a child SP' weighs more than a very thin adult person P. Further, it is impossible to judge whether the status of a little child on the passenger seat is RFIS or FFCS.
By the second method employing image processing, it is possible to judge rather precisely whether or not a passenger is seated on the passenger seat, whether the passenger is an adult P or a child SP' or an infant SP, and whether the status of a little child on the passenger seat is RFIS or FFCS. However, image processing and pattern matching against various kinds of patterns have to be executed to image data obtained by a camera, and thus complex and expensive image processing device is needed.
2. Description of Related Art
In order to resolve the above problems, the present inventors have lately proposed a passenger detection system shown in FIG. 4A through FIG. 8 (Japanese Patent Application No.HEI9-42650). The system utilizes disturbance in a weak alternating electric field which is generated between two electrodes placed on a seat. Referring to FIG. 4A, an oscillator for generating high frequency low voltage is connected to an electrode E1, and another electrode E2 is grounded. An alternating electric field is generated between the electrodes E1 and E2 according to the potential difference between the electrodes E1 and E2, thereby a displacement current Id occurs between the electrode E2 and the ground. In this situation, if an object OB is placed in the electric field as shown in FIG. 4B, the electric field is disturbed by the object OB and thereby the displacement current Id varies into Id1. Almost every object OB can be represented by a conductance and a capacitance, and the object OB is regarded to be connected to the ground via the capacitance.
As shown above, the displacement current varies depending on whether or not an object OB exists on a seat of a car, and the status of a passenger on the seat can be detected by utilizing the phenomenon. Especially, a lot of information about an object on the seat including a passenger can be obtained by increasing the number of electrodes which are placed on the seat, thereby precise detection of the situation on the seat can be executed.
In the following, a concrete example of a passenger detection system utilizing the phenomenon will be described referring to FIG. 5 through FIG. 8. FIG. 5 is a perspective view of a passenger seat which is provided with the passenger detection system which has been proposed by the present inventors. A plurality of electrodes are placed on the upper side of the passenger seat 1. Concretely, electrodes E1 and E2 of quadrangular shapes for example are placed apart on the cushion section 1a, and electrodes E3 and E4 of almost the same shapes are placed apart on the back section 1b. The electrodes E1.about.E4 are formed of electrically conductive fabrics in consideration of comfort of the passenger. However the electrodes E1.about.E4 can also be formed by weaving stringy metal in fabric which covers the seat, by applying electrically conductive paint on fabric which covers the seat, etc., or it is also possible to form the electrodes E1.about.E4 by metal plates. The electrodes E1.about.E4 are connected to a circuit which is shown in FIG. 6 and FIG. 7.
Referring to FIG. 6, the passenger detection system comprises an oscillator circuit 10 for generating high frequency low voltage (for example, 100 Khz and 10.about.12V), a loading current detection circuit 11, a transmission/reception switching circuit 12, a current-voltage converter circuit 13 provided with amplification capability, a detection circuit (demodulation circuit) 14 provided with band passing (unnecessary noise reduction) capability and AC-DC converting capability, an amplification circuit 15, an offset switching circuit 16, and a control circuit 17 such as an MPU which is connected with an air bag unit 18.
FIG. 7 is a circuit diagram showing more concrete details of FIG. 6. In the passenger detection system of FIG. 6 and FIG. 7, the amplification circuit 15 is composed of a first amplification circuit 15A whose gain is .times.1 and a second amplification circuit 15B whose gain is .times.100, and an analog selection circuit 19 is provided for selecting one of the outputs of the first and the second amplification circuits 15A and 15B according to control of the control circuit 17.
The loading current detection circuit 11 is, for example, composed of an impedance device such as a resistor 11a which is inserted to the circuit in series and an amplifier 11b for amplifying the terminal voltage of the resistor 11a, and a current supplied from the oscillator circuit 10 to a particular selected electrode (i.e. the loading current) is detected by the loading current detection circuit 11. The transmission/reception switching circuit 12 is composed of, for example, switching means Aa.about.Ad for connecting the output of the oscillator circuit 10 to an electrode which is selected out of the electrodes E1.about.E4 (hereafter referred to as a `transmission electrode`) and switching means Ba.about.Bd for connecting electrodes other than the transmission electrode (hereafter referred to as `reception electrodes`) to the current-voltage converter circuit 13, in which switching of the switching means Aa.about.Ad and Ba.about.Bd is controlled by the control circuit 17. Incidentally, it is preferable that the transmission/reception switching circuit 12 is composed of a multiplexer circuit. The current-voltage converter circuit 13 is composed of, for example, impedance devices such as resistors 13a for converting the displacement current passing through the reception electrodes into voltages and amplifiers 13b for amplifying the converted voltages, in which a resistor 13a and an amplifier 13b are provided corresponding to each of the electrodes E1.about.E4. The analog selection circuit 19 is composed of, for example, four switching means 19a for being switched simultaneously and connecting the outputs of the second amplification circuit 15B to the control circuit 17 and four switching means 19b for being switched simultaneously and connecting the outputs of the first amplification circuit 15A to the control circuit 17.
FIG. 8 is a circuit diagram showing an example of a circuit which is employed in the air bag unit 18. The circuit of FIG. 8 is basically the same as the circuits of FIG. 1 and FIG. 3, except that the control circuit CC is connected with the control circuit 17 of the circuit of FIG. 6 and FIG. 7.
In the following, the operation of the passenger detection system of FIG. 4A through FIG. 8 will be described. First, according to signals from the control circuit 17, only the switching means Aa in the transmission/reception switching circuit 12 is closed in order to connect the output of the oscillator circuit 10 to the electrode E1, and the switching means Bb.about.Bd are closed in order to connect the electrodes E2.about.E4 to the current-voltage converter circuit 13. Thus, the high frequency low voltage is applied to the transmission electrode E1 by the oscillator circuit 10, and thereby the displacement currents occurs in the reception electrodes E2.about.E4. The displacement currents of the reception electrodes E2.about.E4 are converted into voltages by the resistors 13a and amplified by the amplifiers 13b, and the amplified voltages are supplied to the detection circuit 14. Meanwhile, the loading current passing through the transmission electrode E1 is detected by the loading current detection circuit 11, and the result is supplied to the detection circuit 14 as data R(1,1) which will be explained later. In the detection circuit (demodulation circuit) 14, signal components of the amplified voltages around 100 KHz for example are band passed and unnecessary noise components are rejected, and output signals of the detection circuit 14 are supplied to the first and the second amplification circuits 15A and 15B. Signals from one of the amplification circuits 15A and 15B are properly selected by the operation of the offset switching circuit 16 and the analog selection circuit 19, and the selected signals are supplied to the control circuit 17. For example, when the output signals from the detection circuit 14 can be measured using full-range of the control circuit 17, only the four switching means 19b are simultaneously closed in order to supply the output signals of the first amplification circuit 15A (.times.1) to the control circuit 17. On the other hand, when the output signals from the detection circuit 14 are so small that subtle variations of the output signals can not be measured using full-range of the control circuit 17, only the four switching means 19a are simultaneously closed in order to supply the output signals of the second amplification circuit 15B (.times.100) to the control circuit 17. In the control circuit 17, output signals from the amplification circuit 15A or 15B are A/D converted and stored 30 in memory.
Subsequently, according to signals from the control circuit 17, only the switching means Ab in the transmission/reception switching circuit 12 is closed in order to connect the output of the oscillator circuit 10 to the electrode E2, and the switching means Ba, Bc and Bd are closed in order to connect the electrodes E1, E3 and E4 to the current-voltage converter circuit 13. Thus, the high frequency low voltage is applied to the transmission electrode E2 by the oscillator circuit 10, and thereby the displacement currents occurs in the reception electrodes E1, E3 and E4. The displacement currents of the reception electrodes E1, E3 and E4 are converted into voltages by the resistors 13a and amplified by the amplifiers 13b, and the amplified voltages are supplied to the detection circuit 14. Meanwhile, the loading current passing through the transmission electrode E2 is detected by the loading current detection circuit 11, and the result is supplied to the detection circuit 14 as data R(2,2) which will be explained later. Output signals from the detection circuit 14 are processed in the same way as above and the processed data are stored in memory of the control circuit 17.
Subsequently, only the switching means Ac is closed in order to connect the output of the oscillator circuit 10 to the electrode E3, and the switching means Ba, Bb and Bd are closed in order to connect the electrodes E1, E2 and E4 to the current-voltage converter circuit 13. Thus, the high frequency low voltage is applied to the transmission electrode E3 by the oscillator circuit 10, and thereby the displacement currents occurs in the reception electrodes E1, E2 and E4. The displacement currents of the reception electrodes E1, E2 and E4 are converted into voltages by the resistors 13a and amplified by the amplifiers 13b, and the amplified voltages are supplied to the detection circuit 14. Meanwhile, the loading current passing through the transmission electrode E3 is detected by the loading current detection circuit 11, and the result is supplied to the detection circuit 14 as data R(3,3) which will be explained later. Output signals from the detection circuit 14 are processed in the same way as above and the processed data are stored in memory of the control circuit 17.
Subsequently, only the switching means Ad is closed in order to connect the output of the oscillator circuit 10 to the electrode E4, and the switching means Ba, Bb and Bc are closed in order to connect the electrodes E1, E2 and E3 to the current-voltage converter circuit 13. Thus, the high frequency low voltage is applied to the transmission electrode E4 by the oscillator circuit 10, and thereby the displacement currents occurs in the reception electrodes E1, E2 and E3. The displacement currents of the reception electrodes E1, E2 and E3 are converted into voltages by the resistors 13a and amplified by the amplifiers 13b, and the amplified voltages are supplied to the detection circuit 14. Meanwhile, the loading current passing through the transmission electrode E4 is detected by the loading current detection circuit 11, and the result is supplied to the detection circuit 14 as data R(4,4) which will be explained later. Output signals from the detection circuit 14 are processed in the same way as above and the processed data are stored in memory of the control circuit 17.
Then, the control circuit 17 calculates the seating pattern on the passenger seat 1 by executing arithmetic logic operation to the data. Various types of seating patterns are prestored in the control circuit 17, and a seating pattern which has been calculated using various combinations of a transmission electrode and reception electrodes chosen from the electrodes E1.about.E4 is compared with the prestored seating patterns and one or more matched seating patterns are extracted from the prestored seating patterns in order to judge the status of the passenger on the passenger seat 1. The control circuit 17 regards the following typical seating patterns as objects of matching, for example, a `vacant seat pattern` in which no passenger is seated on the passenger seat 1, a `FFCS pattern` in which a child is seated on the passenger seat 1 in FFCS, a `RFIS pattern` in which an infant is seated on the passenger seat 1 in RFIS, and a `person pattern` in which an adult person is seated on the passenger seat 1. By various combinations of a transmission electrode and reception electrodes chosen from the electrodes E1.about.E4, a plurality of data which are generally represented as R(i, j) can be obtained. Here, R(i, j) in which i=j is transmission data, and R(i, j) in which i . j is reception data in which i and j are representing a transmission electrode and a reception electrode respectively. The control circuit 17 executes arithmetic logic operation using 16 pieces of data R(i, j) for example, and extracts characteristics of the seating pattern.
Then, a signal according to the seating pattern determined by the control circuit 17 is transmitted by the control circuit 17 to the air bag unit 18. For example, a signal instructing the air bag unit 18 to set itself at the no deployment mode (in which the air bag unit 18 does not deploy the air bag for the passenger seat 1 on collision) is transmitted by the control circuit 17 in the case where the determined seating pattern is the vacant seat pattern, the FFCS pattern, or the RFIS pattern, and a signal instructing the air bag unit 18 to set itself at the deployment mode (in which the air bag unit 18 deploys the air bag for the passenger seat 1 on collision) is transmitted by the control circuit 17 in the case where the determined seating pattern is other than the above patterns. These signals are received by the control circuit CC of the air bag unit 18, and in the former case, the control circuit CC is set not to supply a gate signal to the semiconductor switching device SW2 on the side of the passenger seat 1 on collision. Incidentally, the semiconductor switching device SW1 on the side of the driver's seat is necessarily supplied with a gate signal on collision. In the latter case, the control circuit CC is set to supply gate signals to the semiconductor switching devices SW1 and SW2 on collision.
According to the above passenger detection system, a plurality of electrodes E1.about.E4 are placed on the upper side of the passenger seat 1 and a weak alternating electric field due to high frequency low voltage applied between a selected transmission electrode and other reception electrodes is generated, and displacement currents depending on a seating pattern of the passenger on the passenger seat 1 pass through the reception electrodes. Therefore, the seating pattern of the passenger on the passenger seat 1 can be correctly detected by analyzing characteristic patterns in the displacement currents, and thereby the air bag unit 18 can be set at the no deployment mode or the deployment mode according to the seating pattern of the passenger on the passenger seat 1.
Further, the number of the electrodes placed on the passenger seat 1 can be arbitrarily increased and the number of combinations of transmission electrodes and reception electrodes can be increased in order to increase obtained data and execute more precise judgment of the seating pattern of the passenger on the passenger seat 1.
Furthermore, a large number of displacement current patterns corresponding to the `empty pattern`, the `RFIS pattern`, the `FFCS pattern`, `person pattern`, etc. corresponding to each combination of the transmission electrode and the reception electrodes can be stored in the control circuit 17 as the seating patterns. Therefore, actual seating pattern can be detected precisely by use of various combinations of transmission electrodes and reception electrodes and extracting a most probable seating pattern by executing pattern matching.
However, these days, there exist cases where only a function of detecting whether or not a passenger is seated on the passenger seat is required of passenger detection systems, depending on the type of a car. If the above passenger detection system capable of detecting the `empty pattern`, `RFIS pattern`, `FFCS pattern`, `person pattern`, etc. is installed on a car which only requires detection of whether or not a passenger is seated on the passenger seat, only a limited function of the passenger detection system would be used by the car, and thus it becomes overqualification or waste of cost.
It is of course possible to employ a weight sensor as the passenger detection sensor device SD in FIG. 3 for detecting whether or not a passenger is seated on the passenger seat, and cost for the air bag system can be reduced. However, the weight sensor can only detect the weight of one or more articles on the passenger seat, and thus can not discriminate between a passenger and an article other than a passenger. Therefore, other means has to be adopted.